iMac is a locked ecosystem. If you have the means, and the money, CPU and GPU upgrade might be worthwhile because it cannot be replaced later, or at least not as easily as storage or RAM, for example.
Yeah you paid premium for it, you see an upgrade from stock i5 to i7 costs $250 while the real-world difference is only about $100. Why would I pay $250 for an i7 UPGRADE and for which I don't get the stock i5? The same case for graphic option, Apple asks $250 to switch from M290x to M295x, which is also ridiculous. And there is also shameless Apple RAM upgrade for the blind and lazy consumers. But then again you're buying a sealed disposable machine, so you're really at Apple's mercy here.
If you only do facebook, itunes or try to impress some friends though, even the base $2500 retina iMac is a lot of money to spend for a computer, let alone CTO ones.